Apparatus and method for suppressing diffraction rings in a laser

ABSTRACT

A laser comprises an interaction region formed by a wall surface of a capillary discharge tube which has been roughened along a continuous length thereof. This roughened surface suppresses and substantially eliminates diffraction rings in lasers.

This application is a continuation, of application Ser. No. 108,550,filed 10/14/87 now U.S. Pat. No. 4,827,484.

BACKGROUND OF THE INVENTION

The invention relates to lasers and, particularly, to an apparatus andmethod for suppressing diffraction rings in lasers.

Presently available commercial lasers produce undesirable rings of lightin their output beams. These rings are commonly referred to as"diffraction rings" and appear as halos of light forming concentriccircles about the central laser spot. When lasers are used as opticalscanners, e.g., UPC bar code readers, it is important to have as sharp abeam as possible. Diffraction at the output can be construed as multiplebeams and may result in false readings.

SUMMARY OF THE INVENTION

The present invention comprises a laser for producing an output beam ofoptical radiation. The laser includes a pair of reflectors which form anoptical cavity having an optical path for propagating light within theoptical cavity. A laser medium is provided within the resonant cavityfor amplification of the light propagating along the optical path bystimulated emission of photons through an interaction region. Theinteraction region is bounded by a surface which extends longitudinallyalong the optical path, such that the longitudinal axes of theinteraction region and the optical path are substantially coincident. Inthe embodiment disclosed, the surface of the interaction region isroughened along a continuous length thereof. Such roughening inhibitsthe reflection of the light from the surface, such that the diffractionrings in the output beam are substantially eliminated.

In the preferred embodiment, the laser of the present inventioncomprises an elongate tubular envelope for containing a laser gas. Thelaser also includes an anode and a cathode, both of which are disposedin communication with the laser gas. A discharge current is generated inthe laser gas by applying a potential difference between the anode andcathode, for example, by means of a DC power supply.

The cathode is tubular and includes a forward cathode region and arearward cathode region. In the rearward cathode region, the cathode ispreferably cylindrical and has a substantially uniform diameter. In theforward cathode region, the cathode has a generally hemisphericalconfiguration.

A capillary discharge tube is disposed within the envelope. The tubeincludes a bore for providing a path for conducting discharge currentfrom the anode. This tube has a first opening adjacent the anode forplacing the bore in communication with the anode, and additionally has asecond opening for discharging current from the bore to the cathodealong a bore to discharge path. In the preferred embodiment, the bore isroughened by means of a sandblasting unit.

The roughened bore substantially eliminates diffraction rings in thelaser output. As mentioned above, diffraction rings are halos of lightwhich form concentric circles around the central laser beam spot. It hasbeen found that these rings are the result of stray light particlesreflecting off the interior surface of the capillary discharge tube andinterfering with the mainstream of light.

By roughening the interior surface of the capillary discharge tube,light incident thereon tends to scatter rather than reflect. Thisscattering of the light particles lessens the amount of interferencewith the main beam, and thus the diffraction ring problem issubstantially eliminated.

As the roughness of the bore increases, the diffraction ring intensitydecreases. To achieve significant reduction in diffraction ringintensity, the average surface roughness should be one micron or more.However, it is preferable that the average bore roughness be more thanfour microns, and in the preferred embodiment, such roughness is 4.640microns.

The laser of the preferred embodiment utilizes helium neon gas and maybe adapted to operate at various wavelengths. The concepts underlyingthe present invention are widely applicable to any laser havingdiffraction ring problems, including solid lasers, and other types ofgas lasers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the laser of the preferred embodiment;

FIG. 2 is a perspective view of the laser of FIG. 1, with a portion ofthe envelope and cathode cut away to illustrate the mounting arrangementof the capillary discharge tube;

FIG. 3 is an elevational view, in partial cross-section, of the anodeend portion of the envelope, illustrating the mirror alignment collarutilized at both the anode and cathode ends of the laser for selectivelydeforming the mirror mount to adjust the orientation of the mirrors forreflecting light therebetween to form the optical cavity of the laser;

FIG. 4 is a schematic drawing illustrating an output laser beam with atypical diffraction ring pattern;

FIG. 5 is a graph showing the energy distribution pattern for the firstorder mode of the laser;

FIG. 6 is a schematic representation of the laser of the presentinvention in operation, showing the main beam of light and stray rayswhich reflect off the surface of the bore within the capillary dischargetube and interfere with the main beam;

FIG. 7 is a schematic drawing illustrating the use of a sandblastingunit to roughen the bore of the capillary discharge tube;

FIG. 8 is a schematic drawing illustrating the use of a contactprofilometer to measure the average bore roughness over a continuouslength within the capillary discharge tube; and

FIG. 9 is a schematic representation of a rod of solid laser materialsuch as YAG, in which the exterior surface of the laser rod has beenroughened to substantially eliminate diffraction rings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The laser 100 of the preferred embodiment comprises a gas laser having a5 milliwatt output. As shown in FIG. 1, the laser 100 comprises anelongate tubular envelope 102 having an anode end portion 104 and acathode end portion 106. The envelope 102 is cylindrical and is formedof an insulating material, such as silica glass. In the preferredembodiment, the envelope 102 is on the order of about a foot in length.

Referring to FIGS. 1 and 2, an anode 108 is sealed to the anode end 104of the envelope 102. The anode 108 is electrically connected and sealedto a mirror mount 110. A planar mirror 112 is mounted on and sealed tothe mirror mount 110, e.g., by means of a solder glass. The cathode end106 portion of the envelope 102 is sealed to an end plate 114 which, inturn, is electrically connected and sealed to a mirror mount 116. Aspherical mirror 118 is mounted on and sealed to the mirror mount 116,e.g., by means of a solder glass. The mirrors 112, 118, mirror mounts110, 116, anode 108 and end plate 114 cooperate to seal the envelope 102to provide an enclosed volume for containing a laser gas which, in thepreferred embodiment, comprises helium neon gas.

A tube 120 which passes through and is sealed to an aperture 122 in theend plate 114 is provided for the purposes of evacuating air from theenvelope 102 and filling the envelope 102 with the helium neon gas. Oncethe envelope 102 has been filled with the laser gas, the tube 120 isclosed, e.g., by crimping.

In the embodiment disclosed, the helium neon gas is comprised of He³,Ne²⁰ and Ne²² in a 17-to-1-to-1 ratio of helium to neon. Those skilledin the art will understand that other gas mixtures may be usedalternatively.

An elongate capillary discharge tube 124 is disposed within the envelope102. This capillary discharge tube 124 has a central longitudinal bore126 which has a uniform diameter and extends through the entire lengthof the capillary discharge tube 124. The capillary discharge tube 124 isoriented such that the central longitudinal axis of the bore 126 iscoincident with the central longitudinal axis of the envelope 102. Atthe anode end 104 of the envelope 102, the capillary discharge tube 124is joined to the envelope 102 portion so that it is cantilever supportedthereby. Further, the tube 124 has an opening (not shown) adjacent tothe anode 108 so that the laser gas is in communication with the anode108. This opening is formed by an enlarged diameter tube portion, whichis in communication with the bore 126. The capillary discharge tube 124extends from the anode end 104 towards the cathode end 106, and isformed of a non-metallic, insulating material, such as glass.

At the end of the tube 124, an opening 128 is provided in the bore 126to allow discharge current from the anode 108 to flow out of the tube124. In the preferred embodiment, the discharge tube 124 is about 11inches in length. However, the length may vary depending on factors suchas the output power of the laser. For example, in an alternativeembodiment, the discharge tube may be about 3-4 inches in length. Thebore 126 is sized for single mode operation in the preferred embodiment,and has a diameter of about 1/16 inch.

The laser 100 also includes a tubular cathode 130 at the cathode end 106of the envelope 102. The cathode 130 is a tubular elongate, cylindricalsymmetric structure having a central longitudinal axis which iscoincident with the central longitudinal axis of the envelope 102. Thoseskilled in the art will understand that a cylindrical symmetricstructure is a structure in which the radial distance at any point alongthe longitudinal axis is the same. The cathode 130 in the preferredembodiment includes a cylindrical portion 132 having a uniform diameterselected such that the outer surface of the cylindrical portion 132 isadjacent to the inner surface of the cathode end 106 of the envelope 102along its length. The cathode 130 also includes a second generallycylindrical portion 134 which has a reduced diameter relative to thefirst cylindrical portion 132. Preferably, this portion 134 also has auniform diameter. This cathode portion 134 is sized to fit within acentral aperture 136 formed in the end plate 114. The end plate 114 hasa flanged portion which forms a collar 138 around the cathode portion134 to provide good electrical contact therebetween. The flanged collar138, in turn, fits within a central longitudinal bore 140 in the mirrormount 116 such that the end plate 114 and cathode portion 134 makeelectrical contact with the mirror mount 116. Interposed between thelarger diameter cylindrical portion 132 and the reduced diametercylindrical cathode portion 134 is a generally hemispherical cathodeportion 142 which provides a transition therebetween. The hemisphericalportion 142 is disposed such that the center of the hemisphere islocated in proximity to the center of the opening 128. The reduceddiameter cylindrical portion 134 and the hemispherical cathode portion142 form a "snout-like," funnel-shaped structure which closes off asubstantial portion of one end of the cathode 130.

For the purpose of reference hereinafter, the envelope is divided into aforward envelope region and a rearward envelope region. The forwardenvelope region comprises the portion of the envelope 102 between theopening 128 in the capillary discharge tube 124 and the cathode end 106.The rearward envelope region comprises the portion of the envelope 102between the opening 128 and the anode end 104. Thus, the laser 100 maybe viewed as being divided into two parts by an imaginary plane normalto the central longitudinal axis, disposed at the opening 128. Thisplane also divides the cathode 130 into a forward cathode region,disposed in the forward envelope region and a rearward cathode regiondisposed in the rearward envelope region.

The capillary discharge tube 124 extends into the cathode 130 such thatthe opening 128 of the bore 126 is disposed substantially at thejuncture of the larger diameter cathode portion 132 and the transitionalor hemispherical cathode portion 142. Thus, the cathode 130 surroundsthe capillary discharge tube 124 exclusively through the length of thelarge diameter cathode portion 132. Depending on the overall length ofthe capillary discharge tube 124, it may be preferable to support thecapillary discharge tube 124 intermediate its ends, e.g., by means of aspider structure 144, such that the capillary discharge tube 124 extendstoward the cathode end 106 from the spider 144 in cantilever fashion.Attached to the spider 144, and extending into the rearward enveloperegion is a getter 145.

The mirror mount 110 at the anode end 104 of the envelope 102 includes acentral longitudinal bore (not shown), similar to the centrallongitudinal bore 140 of the mirror mount 116 at the cathode end 106.The central longitudinal axes of these bores are coincident with thecentral longitudinal axis of the envelope 102, so as to form an opticalpath through the laser 100 between the mirrors 112, 118.

Those skilled in the art will understand that to form an optical cavity,the mirrors 112, 118 must be properly aligned. To facilitate suchalignment, the laser 100 of the present invention includes a pair ofmirror alignment collars 146, 148. For the purposes of illustration,only the collar 146 on the mirror mount 110 is shown in FIG. 3.

The collar 146 is disposed on he exterior of the mirror mount 110adjacent an annular slot 150 formed in the exterior surface of themirror mount 110. The collar 146 includes a series of set screws 152spaced around the collar 146, e.g., at 120° intervals, such that the setscrews 152 may be driven into the annular slot 150. The set screws 152are slightly larger than the slot 150 and have a conically tapered end.Thus, by driving one of the set screws 152 into the slot 150, theconical end of the set screw 152 will spread the portion of the slot 150adjacent thereto, while causing corresponding narrowing of the slot 150on the opposite side thereof. Accordingly, by manipulating selected onesof the set screws 152, the orientation of the mirror 112 may be properlyadjusted. Additionally, the use of a plane mirror 112 at the anode end104 and a spherical concave mirror 118 at the cathode end 106facilitates the alignment of the mirrors 112, 118, forming the opticalcavity in the laser, and provides a very stable arrangement.

The principles of laser operation are well known in the art and thuswill be only be briefly described. The laser 100 is energized byapplying a potential difference (voltage) between the anode 108 and thecathode 130. Since the anode 108 is electrically connected to the mirrormount 110 and the cathode 130 is electrically connected to the mirrormount 116, the mirror mounts 110, 116 may serve as terminals forapplication of the potential difference. In the preferred embodiment,the potential difference or voltage is generated by a DC power supply(not shown). Application of such voltage between the anode 108 and thecathode 130 causes ionization of the laser gas within the envelope 102.Accordingly, a discharge current flows from the anode 108 through thebore 126 of the capillary discharge tube 124 to the inner surface of thecathode 130. Conversely, electrons from the cathode material flowthrough the laser gas to the bore opening 128 and through the bore 126to the anode 108.

The discharge current excites atoms of laser gas within the bore 126 toa higher energy state in accordance with well known laser principles toprovide an active "pumped" gain medium. The excited atoms then relax toa lower energy level, during which time they emit photons (lightparticles) having a wavelength characteristic of the difference betweenthe energy levels. Although laser gases typically emit such photons at avariety of wavelengths, the laser 100 may be adapted to causepreferential emission at a particular wavelength, for example, 632.8 nm.One common method involves coating the mirrors 112, 118 with areflective coating which preferentially reflects light at the desiredwavelength. The reflected photons cause emission of additional photonsat the same wavelength in accordance with a phenomena commonly referredto as "stimulated emission." This causes light of the desired wavelengthto preferentially build up in the optical cavity of the laser 100 andthereby stimulates emission from the laser gas at the desiredwavelength.

Referring to FIG. 4, the laser output beam forms a central beam spot 156having a circular cross section. As discussed previously, undesirablediffraction rings 155 can form around the central laser beam spot 156,as illustrated in FIG. 4 by phantom lines.

An energy distribution pattern 162 for the first order, or fundamentalmode of the laser, is shown in FIG. 5. A Y-axis 158 represents theintensity of the light, while an X-axis 160 represents the distance fromthe center of the bore 126. As illustrated, the energy distributionpattern 162 of the fundamental mode is Gaussian shaped, such that mostof the light intensity is concentrated at the center 164 of the pattern162. A central area of the energy distribution curve 162, whichcorresponds to the bright central output beam spot 156 (FIG. 4) isdefined by lines 166, 168. Light outside of these lines 166, 168,referred to herein as tail sections 170, 172, is of a much lesserintensity.

As shown in FIG. 6, when the laser 100 is operating, light propagates inan optical path 174, which extends between the mirrors 112, 118. Becausethe mirror 112 is planar while the mirror 118 is curved, the light formsa more or less conically shaped optical beam 176 which tends to divergeand converge.

As the light propagates back and forth between the mirrors 112, 118, thelight is amplified by stimulated emission. Such stimulated emissionoccurs in an elongated interaction region 177, which has a longitudinalaxis 178, substantially coincident with the optical path 174. Thisinteraction region 177 is bounded on sides thereof by the non-metallicinner walls 180 of the discharge tube 124, and in the embodiment shown,extends the full length of the bore 126. Stated another way, theinteraction region 177 includes the entire cylindrical volume within theinner walls 180 of the discharge tube 124, so that the inner wallsurface 180 of the discharge tube 124 is thus in communication with thelaser gas plasma along the entire length of the interaction region 177.The interaction region 177 thus contains and confines the gas plasmaassociated with the laser discharge.

As the light particles are reflected between the mirrors 112, 118, mostof the light intensity is concentrated about the central axis 178 of theinteraction region 177, and this light forms the center spot 156.However, some of the light corresponding to the tail sections 170, 172on the energy distribution curve 162 will impinge on the bore surface180, i.e., the inner walls of the capillary discharge tube 124. If thelight, incident on the surface 180 of the capillary discharge tube 124,is at an angle known as the "grazing angle," the light will be totallyinternally reflected. Upon reflection, the reflected light 182 willtraverse an optical path which is different from that of the main beam156. This reflected light 182 can interfere with the main beam 156 oflight and results in the diffraction ring pattern 155 seen at theoutput.

In the laser 100 of the preferred embodiment, the interior surface 180of the capillary discharge tube 124 is roughened throughout the centrallongitudinal bore 126 to provide a uniformly textured surface. Thisroughening the bore 126 of the capillary discharge tube 124, throughoutthe length of the interaction region 177, tends to scatter the lightparticles incident thereon, rather than reflect them. By scattering thelight particles, less light interferes with the main beam 156; thus thediffraction rings 155, as seen at the output, are greatly reduced inintensity and substantially eliminated.

In the preferred embodiment, the interior surface 180 of the capillarydischarge tube 124 is roughened by an abrasive "sandblasting" unit 184,as shown in FIG. 7. By way of example, an AIRBRASIVE UNIT MODEL HME,commercially available from S. S. White Industrial Products, Piscataway,N.J. Prior to final assembly of the laser 100, an output nozzle 186 ofthe unit 184 is abutted against the anode and end of the capillarydischarge tube 124 and the capillary discharge tube 124, in turn, actsas an extension of the nozzle 186. An aluminum oxide grit 188, having adiameter, for example, of 150 μ, is shot into the bore 126 for apredetermined time at a predetermined pressure, for example, at 120p.s.i. for ten seconds. Preferably, the nozzle 186 should be positionedsuch that it is longitudinally aligned with the central longitudinalaxis of the capillary discharge tube 124 to ensure a substantiallyuniform roughening throughout the bore 126. Alternatively, the bore 126can be roughened by using a valve grinding compound and turning the bore126 on a mandril.

Referring to FIG. 8, a contact profilometer 190 may be used to measurethe degree of roughness within the bore 126. As is well known, theprofilometer 190 utilizes a stylus 192 in direct contact with thesurface 180 of the bore 126. The peaks and valleys on the bore surface180 are measured over a path of at least 2 mm, and an average boreroughness is calculated. Tests indicate that as the bore textureincreases in roughness, the diffraction ring intensity decreases.

It has been determined that, to achieve significant reduction indiffraction ring intensity, the average surface roughness should be onemicron or more. However, it is preferable that the average boreroughness be more than 4 microns. In the preferred embodiment, suchroughness is 4.640 microns. Additionally, it is preferable that the boresurface 180 be roughened over substantially the entire length of theinteraction region 177 to prevent light rays 182 incident on any portionthereof from propagating along the optical path 174 and interfering withlight in the main output beam 156.

Although the preferred embodiment utilizes a 5 milliwatt gas laser, itshould be recognized that the present invention is applicable to lasersof virtually any power level. Additionally, while the laser 100described in the preferred embodiment is a HeNe gas laser, thetechniques described herein are applicable to any laser having thediffraction ring problem solved by the present invention, for example,solid lasers, and lasers using gases other than HeNe.

FIG. 9 is a schematic representation of a rod 194 of laser material suchas YAG. Solid lasers are well known in the art and, like gas lasers,operate on the principle of amplification by stimulated emission. Thesolid laser rod 194 provides a laser gain medium for the laser and thelight propagates longitudinally through the rod 194 between mirrors (notshown). The rod 194 forms an interaction region in which the stimulatedemission occurs and this interaction region is bounded by the exteriorside surface of the rod. When the techniques of the present inventionare applied to solid lasers, the exterior side surface of the laser rod194 is roughened. In a matter analogous to the gas laser describedherein, roughening the exterior surface of the laser rod 194 tends toscatter the light which is incident on the exterior surface of the rod194 at the grazing angle. This scattering of the light rays reducesinterference with the main beam, thereby suppressing diffraction rings.

The present invention thus significantly reduces diffraction rings 155in the output 156 of the laser beam and provides a new class of lasershaving output beams 156 which are significantly sharper than those ofpresently available lasers.

What is claimed is:
 1. A method of manufacturing a gas laser having anenvelope for containing a laser gas and a tube in said envelope whichsurrounds an optical path, said method comprising flowing a substancethrough said tube to roughen interior surfaces of said tube such thatdiffraction rings in light output from said laser are suppressed.
 2. Themethod of claim 1, wherein said substance comprises abrasive particles.